1. Field of the Invention
The present invention relates to an anode material for non-aqueous lithium secondary batteries having a large capacity and a small irreversible capacity loss, a method for the production thereof, and to batteries using such anode materials.
2. Description of the Related Arts
Non-Aqueous Lithium Secondary Batteries using carbonaceous materials as the anode as lithium secondary batteries have already been put into use because of the advantages of high energy density, small size and weight and the ability to hold a charge for a long period of time. However, the new increase in charge capacity of the anode use carbonaceous materials is inadequate to accommodate the trends in miniaturization of electrical devices and the lengthening of their period of operation without recharging.
Up until now, as the carbonaceous material used in the anodes of lithium ion secondary batteries, non-graphite type carbonaceous material with a capacity of nearly 500 mAh/g, greatly surpassing graphite materials having a capacity of 370 mAh/g have been reported. For example, materials made by calcining the reaction products of pitch or tar type compounds that have been reacted with nitrocompounds (Japanese Laid Open Patent Application Hei4-258479 [1993]), air oxidation and calcination of the reaction products of pitch or tar type compounds that have been reacted with nitric acid (Japanese Laid Open Patent Application Hei7-320740 [1995]), the calcination of palm shell charcoal at reduced pressures (Japanese Laid Open Patent Application Hei8-64207 [1996]), and materials made by improving pitch obtained by such methods as polymerization of conjugated polycyclic hydrocarbons followed by stabilization and calcination (Japanese Laid Open Patent Application Hei10-083814 [1998]), may be mentioned.
However, the materials made to date have had problems in connection with, in particular, charge and discharge efficiencies, economy, and stability of raw materials and the like. Moreover, materials having a high discharge capacity exceeding 600 mAh/g made by calcining coke at low temperatures or a polyacene made by calcining a phenolic resin (Yada et al.: Abstracts of the 34th Battery Symposium in Japan, p 63, 1993) have been discovered, but the excessive size of the irreversible capacity (the difference in charge capacity and discharge capacity in the first cycle) is a problem. In addition, because the discharge potential is high, there is also the problem that, when assembled with the cathode material to make a battery, the average voltage of that battery is low.
U.S. Pat. No. 5,093,216 to Azuma et al. discloses the use of furfural or a copolymer of furfural with another aldehyde, and an anode material is then prepared from that polymer. While a high initial battery capacity may be exhibited, the irreversible capacity loss of this material in the first cycle was observed to be excessive, equaling as much as one-third of total initial capacity.
As explained above, when the lithium secondary batteries using a non-aqueous solvent incorporate as the anode material the carbonaceous materials of the prior art, the capacity that can be obtained as a characteristic of those materials is not adequate. The present invention provides an anode use carbonaceous material that overcomes the difficulties of the prior art, possesses a high capacity of 500 mAh/g or more, reduces the irreversible capacity loss of the anode use carbonaceous material occurring in the first cycle, and when assembled with the cathode material to make a secondary battery, results in a battery with a high average voltage.
The inventors of the present invention, based upon the results of earnest investigations of high capacity carbonaceous material for anode use using tar or pitch as a raw material, have succeeded in discovering that the carbonaceous material prepared by calcining at 800xc2x0 C. or more the solid obtained by heating pitch and/or tar and furfural in the presence of an acid catalyst differs from that made using tar, pitch or furfural alone and thereby have discovered an anode material possessing superior characteristics for a secondary battery using a non-aqueous solution.
That is to say, the present invention relates to a carbonaceous material for use in the anode of a secondary battery using an aqueous solvent characterized by calcining at a temperature of at 800xc2x0 C. or more of a solid obtained by heating tar and/or pitch with furfural in the presence of an acid catalyst and to a method for making an anode material for use in the anodes of secondary batteries using non-aqueous solvents characterized by calcination at a temperature of at 800xc2x0 C. or more of a solid obtained by heating tar and/or pitch with furfural in the presence of an acid catalyst, and to batteries using such anode materials.